1. Field of the Invention
The present invention generally relates to thermal printing apparatus and, more particularly, is directed to a thermal printing apparatus in which a correct density of printed image can be realized by a set of correction data regardless of a horizontal frequency of a video signal and an aspect ratio.
2. Description of the Prior Art
A conventional thermal printing apparatus produces a video picture as a hard copy printed image according to the following method. This method will be described hereinafter with reference to FIGS. 1A to 1C.
Referring to FIG. 1A, there are shown a thermal print head 1 which is comprised of heating elements R.sub.1 to R.sub.n (m=1280) of one horizontal line, for example, 1280 pixels and in which the heating elements R.sub.1 to R.sub.m are provided in the horizontal direction, and a print paper 2 on which an image is printed. The print paper 2 is continuously transported in the vertical direction relative to the thermal print head 1.
At that time, the print paper 2 is made as a thermal printing paper or the print paper 2 is a standard paper. In the latter case, a thermal print ink ribbon (not shown) is interposed between the thermal print head 1 and the print paper 2.
Pixel data of one horizontal line of a video signal (luminance signal), in this example, pixel data of m number are converted into pulse width modulated (i.e., PWM) signals S.sub.1 to S.sub.m of pulse width Td corresponding to the densities of respective pixels as shown in FIG. 1B. Then, these PWM signals S.sub.1 to S.sub.m are supplied to the heating elements R.sub.1 to R.sub.m, respectively.
Accordingly, pixels P.sub.1 to P.sub.m of m number are simultaneously printed on the print paper 2 at every line by the heating elements R.sub.1 to R.sub.m and, as shown in FIG. 1C, lengths L in the vertical direction of the pixels P.sub.1 to P.sub.m are changed in response to the pulse widths Td of the PWM signals S.sub.1 to S.sub.m, thereby densities of pixels P.sub.1 to P.sub.m being expressed, respectively. In this case, for example, 7 bits are assigned to one pixel and the density or darkness thereof is expressed by 128 gray levels.
The aforenoted operations are carried out for all pixels at every horizontal line, thus the video picture being produced as the hard copy. Although the signals S.sub.1 to S.sub.m are pulse number modulated (PNM) signals, they are described as the PWM signals for simplicity.
FIG. 2 shows an example of a circuit for effecting such hard copy operation.
As FIG. 2 shows, the heating elements R.sub.1 to R.sub.m of the thermal print head 1 and collector-emitter paths of transistors Q.sub.1 to Q.sub.m which drive the heating elements R.sub.1 to R.sub.m are respectively connected in series between a voltage source terminal T.sub.0 and the ground.
A frame memory 11 derives pixel data d.sub.1 to d.sub.m of one horizontal line and the pixel data d.sub.1 to d.sub.m are supplied through a line memory 12 to a converting circuit 13, in which they are converted into data D.sub.1 to D.sub.m, respectively.
In that case, each of the data d.sub.1 to d.sub.m is formed of, for example, 7 bits as described above and the data D.sub.1 to D.sub.m are 128 bits which are equal to the 128 gray levels of densities for the pixel. Of 128 bits, the bits of the number corresponding to the density of pixels from the starting bit are "1" (high level) and the remaining bits are "0" (low level). Therefore, it is to be appreciated that the data D.sub.1 to D.sub.m are the PWM signals (strictly speaking, PNM signals as earlier noted) S.sub.1 to S.sub.m.
Of the data D.sub.1 to D.sub.m thus converted, n'th bits b.sub.1 to b.sub.n (n=1 to 128) are supplied through a latch circuit 14 to the bases of the transistors Q.sub.1 to Q.sub.m, respectively.
Accordingly, the pixels P.sub.1 to P.sub.m are printed on the print paper 2 at every horizontal line by the data D.sub.1 to D.sub.m (signals S.sub.1 to S.sub.m) and, the lengths L in the vertical directions of the pixels P.sub.1 to P.sub.m are respectively changed in response to the pulse number (pulse widths Td of the signals S.sub.1 to S.sub.m) of the data D.sub.1 to D.sub.m, thereby the hard copy of the video picture being obtained as described hereinbefore in FIG. 1.
However, at that time, the print paper 2 is generally white and the densities of pixels are expressed in black by the thermal print head 1 so that, if a relationship between the level of the video signal and the pulse width Td of the PWM signal Si is made linear, the density of the printed image relative to the level of the video signal will not become linear.
To solve this problem, the data d.sub.1 to d.sub.m from the frame memory 11 and passing through the line memory 12 are supplied to a correcting circuit 15, thereby forming correcting data C.sub.1 to C.sub.m. The correcting data C.sub.1 to C.sub.m are supplied to the converting circuit 13, whereby the pulse widths Td of the signals S.sub.1 to S.sub.m are respectively corrected. Thus, the density of the printed image on the print paper 2 is made linear.
In the following description, if the PWM signals S.sub.1 to S.sub.m need not be discriminated from each other, they will be referred to hereinafter as the PWM signal Si.
When the hard copy of the video picture is obtained by using the above-mentioned thermal printing apparatus, it is frequently observed that a problem will occur depending upon a video signal.
That is, a video signal according to the NTSC system has 525 horizontal scanning lines and an aspect ratio of the picture screen is 3:4, whereas a video signal derived from, for example, an X-ray video camera has a different standard from the NTSC video signal.
Further, when a hard copy of a picture of a personal computer is obtained, if the hard copy is obtained under the condition that the picture is rotated on the print paper 2 by 90 degrees, then the short side of the picture screen corresponds to the length direction of the thermal print head 1 so that the size of the printed image can be increased. In that case, the aspect ratio of the picture becomes 4:3 from a printing apparatus standpoint. Furthermore, a so-called high definition television receiver (i.e., HDTV receiver)has a picture screen whose aspect ratio is 9:16.
There are video signals of various types and standards as described above.
Let it now be assumed that, for example, the hard copy of the picture of the NTSC video signal is standard, a pixel Pa in FIG. 3A indicates a pixel printed at that time and that its vertical print pitch Vp is a standard value. Further, a characteristic (standard characteristic) shown by a curve A in FIG. 3B assumes a characteristic of gray level of the video signal relative to the density of a printed image at that time.
When a hard copy of video signal according to the different standard is obtained relative to the above-mentioned standard hard copy, let us assume the following conditions:
The moving speed of the print paper 2: constant
The pulse width Td of the PWM signal Si: constant
The cycle Th of the PWM signal: altered where ##EQU1##
In the case of a certain video signal, it is assumed that the aspect ratio of a printed video image is equal to that of the NTSC video signal and the number of effective horizontal print lines is 3/2 times as the number of effective horizontal print lines of the NTSC video signal.
In that case, the cycle Th of the PWM signal Si must be selected to be 2/3 times the cycle of the NTSC video signal and the vertical print pitch of the pixel Pb of the hard copy must be selected to be 2/3 times the vertical print pitch of the NTSC video signal as shown by the pixel Pb in FIG. 3A, otherwise the aspect ratio of the printed image will become different.
If so, a ratio L/Vp in which the pixel Pb occupies the picture screen in the vertical direction becomes larger than that of the pixel Pa of the NTSC video signal because the length L of the pixel Pb is determined by the pulse width Td of the PWM signal Si and is equal, in that case, to that of the NTSC video signal.
Therefore, the density of printed image of the resultant hard copy is unavoidably increased as shown by a curve B in FIG. 3A.
Further, let it be assumed that other video signal whose aspect ratio is equal to that of the NTSC video signal and that the number of effective horizontal print lines is 1/4 times the NTSC video signal. In that case, the cycle Th of the PWM signal Si must be increased to 4/3 times that of the NTSC video signal and the vertical print pitch Vp of the pixel Pc of the hard copy printed paper must be increased 4/3 times that of the NTSC video signal as shown by the pixel Pc in FIG. 3A, otherwise the aspect ratio of printed image of this video signal is not made correct.
However, if so, the length L of the pixel Pc is determined by the pulse width Td of the PWM signal Si and in this case it is equal to that of the pixel of the NTSC video signal, so that the ratio L/Vp in which the pixel Pc occupies the vertical direction is made smaller than that of the pixel Pa of the NTSC video signal.
As a result, a density of a printed image of the resultant hard copy print paper is decreased as shown by a curve C in FIG. 3B.
In the case of video signal having the same number of horizontal scanning lines as that of the NTSC video signal and whose aspect ratio is different from that of the NTSC video signal, the vertical print pitch Vp thereof is different so that the density of printed image is also changed.
When the standards of the video signals are different as described above, if the hard copy printed paper is obtained under the aforenoted conditions, the density of printed image is fluctuated as shown in FIG. 3B.
Accordingly, when the standard of the video signal is different, the following conditions are proposed:
A moving speed of print paper 2: altered
A pulse width Td of PWM signal Si: constant
A cycle Th of PWM signal Si: constant
According to the above-described conditions, if the aspect ratio of printed image of video signal is equal to that of printed image of the NTSC video signal, although the moving speed of the print paper 2 is changed in response to the number of effective horizontal print lines, the pulse width Td and the cycle Th of the PWM signal Si are constant so that, when the number of effective horizontal print lines of the video signal is 3/2 times that of the NTSC video signal, the pixel printed on the print paper 2 becomes as shown by a pixel Pb in FIG. 4A or that, when the number of effective horizontal print lines of the video signal is 1/4 times that of the NTSC video signal, the pixel printed on the print paper becomes as shown by a pixel Pc in FIG. 4A (pixel Pa in FIG. 4A is the same as the pixel Pa in FIG. 3A).
Accordingly, in that case, the ratios L/Vp between the vertical print pitches Vp.sub.a to Vp.sub.c of pixels Pa to Pc and the lengths L.sub.a to L.sub.c of pixels Pa to Pc are equal to each other regardless of the number of effective horizontal print lines, whereby characteristic curves of gray levels of the video signals and the densities of printed images are all coincident with each other. Therefore, it is appreciated that regardless of the standard and the kind of the video signal, the correct density of printed image can be obtained.
However, the thermal print head 1 has a heat storage capability and the aforenoted equation (i) cannot be established due to the influence of such heat storage capability and the like with the result that, in actual practice, the density characteristics are provided as shown by curves B and C in FIG. 4B and are not coincident with the correct curve A. That is, the correct density characteristic cannot be obtained.
Therefore, when the characteristic curves B and C are not coincident with the correct characteristic curve A as shown in FIGS. 3B and 4B, it may be considered that the characteristic curves B and C are made coincident with the correct characteristic curve A by changing the correction data C.sub.1 to C.sub.m in the correcting circuit 15.
However, if so, the density of printed image is formed of 128 gray levels so that correction data of amount corresponding to the kinds of video signal to be printed.times.128 are required, which unavoidably makes the memory very large in storage capacity for storing the correction data in actual practice Further, it is very cumbersome to form correction data of such large amount.